cleanup: remove key id annotation from user entity

[quexten]

Follow-up cleanup.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This PR removes the [KeyId] validation attribute from the UserKeyId property on the User entity, a follow-up cleanup to the key-id wire-in work in #8164. The attribute was inert on the entity: User is not model-bound in any controller and EF Core does not evaluate ValidationAttributes at runtime, so removal has no behavioral or schema impact. Input validation is still enforced at the API boundary, where [KeyId] remains on the request models (SetUserKeyIdRequestModel, RotateUserKeysRequestModel, MasterPasswordUnlockDataRequestModel, KeysRequestModel, and others), and all internal writes flow through the strongly typed KeyId value object. Remaining using directives in User.cs are still required (CoreHelpers, EncryptionParsing), so no unused-using build warnings are introduced.

Code Review Details

No findings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 15.56%. Comparing base (f956e58) to head (2cf0426).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8191   +/-   ##
=======================================
  Coverage   15.56%   15.56%           
=======================================
  Files        1439     1439           
  Lines       62488    62488           
  Branches     5014     5014           
=======================================
  Hits         9727     9727           
  Misses      52595    52595           
  Partials      166      166           

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